Lean-burn engines are being utilized to improve fuel economy. These engines operate using oxidizing air/fuel ratios and require unique exhaust gas treatment for conversion of the carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into more desirable gases. NOx is the most difficult to convert because the exhaust gas is oxidizing yet NOx needs to be reduced. One of the most promising treatments for lean-burn operation today involves the use of nitrogen oxide absorbent material with precious metal. Such NOx trap materials are able to absorb nitrogen oxides during lean-burn operation and are able to release them when the oxygen concentration in the exhaust: gas is lowered. Hence, these traps are used with engine systems which operate primarily in a lean air/fuel ratio, but then when it is desired to purge the traps of NOx, the exhaust entering the trap is made richer, particularly rich of stoichiometric.
Typical conventional NOx traps use an alkaline earth metal like barium combined with a precious metal catalyst like platinum deposited on a porous support like alumina, as disclosed, for example, in EP patent application 0 613 714. The widely held mechanism for this absorption phenomena is that during the lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the other material, e.g., the barium. In the regeneration mode as during a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released. NOx then catalytically reacts over the platinum with reducing species in the exhaust gas like HC and CO to form O.sub.2 and N.sub.2.
The alkali metal and alkaline earth metals loaded on the porous support as typically utilized for NOx sorption have the serious drawback, however, that they are readily poisoned by sulfur in the exhaust gas. Most fuels for automotive vehicles contain sulfur and when burnt, the sulfur is converted to sulfur compounds like SO.sub.2. Over time, the sulfur compounds react with these alkali metal or alkaline earth trap materials forming sulfates and sulfides which will not easily revert back to the sorption material. These sulfates are inactive for NOx sorption. The alkali metals are particularly problematic.
The regeneration of NOx traps after sulfur poisoning needs to be carried out at relatively high temperatures in order to decompose the sulfates. These high temperatures result in the sintering of platinum as well as alkali metal/alkaline earths-alumina materials. Sintering of platinum means that the small particles of platinum coalesce to form large particles, thereby, reducing the number of catalyst sites. This leads to reduced efficiency of NOx to NO.sub.2 conversion. The sintering of alkali metal/alkaline earths-alumina materials means that the these materials loose surface area due to increase in particle size and loss of porosity. This results in inefficient contact with platinum and exhaust gases leading to inefficient trapping.
In commonly owned application Ser. No. 09/184,146 filed Nov. 2, 1998 and entitled "Use of Sol-Gel Processed Alumina-Based Metal Oxides for Absorbing Nitrogen Oxides in Oxidizing Exhaust Gas", we disclose use of a precious metal/oxide material for absorbing NOx in lean-burn engine exhaust. The sol-gel processed oxide material includes oxides of metals, consisting essentially of aluminum; at least one of alkali metal and alkaline earth metal; and optionally lanthanide metal. The material can be made from heterometallic alkoxides.
According to the present invention, we have now found that an aluminum based oxide substantially free of precious metal and made by sol-gel techniques from alkoxides including heterometallic alkoxides provides NOx traps which have excellent thermal stability, being significantly better than conventional NOx traps. The present invention aluminum based oxide includes transition metal. In addition, it includes alkali metal, alkaline earth metal, or both. In the oxides, chemical bonds exist between the elements therein as compared to traditional alumina materials which have been loaded with metals which are devoid of such chemical bonds. Ir the invention oxide, the transition metals exist as fully formed metal oxides under lean burn conditions and are in intimate contact with alkali metal/alkaline earths in alumina matrix. Thus the transfer and entrapment of NOx occurs efficiently. High temperature treatment to remove SOx which may have been absorbed does not reduce the ability of transition metal oxides towards later NOx conversion.